ESTROGEN-INDUCED C-FOS PROTOONCOGENE EXPRESSION IN MCF-7 HUMAN BREAST-CANCER CELLS - ROLE OF ESTROGEN-RECEPTOR SP1 COMPLEX-FORMATION

17 beta-Estradiol (E-2) induces c-fos protooncogene expression in MCF- 7 human breast cancer cells, and previous studies in HeLa cells identi fied an imperfect palindromic estrogen-responsive element (-1212 to -1 200) that was required for trans-activation. In contrast, the estrogen -responsive element was not required for E-2 responsiveness in MCF-7 c ells, and using a series of constructs containing wildtype (pF1) and m utant 5'-flanking sequences (-1220 to -1155) from the c-fos protooncog ene promoter in transient transfection assays, it was shown that a GC- rich motif (5'-GGGGCGTGG) containing an imperfect Sp1-binding site was required for hormone-induced activity. This sequence also bound Sp1 p rotein in gel mobility shift assays, and coincubation with the estroge n receptor (ER) enhanced Sp1-DNA binding. E-2 and 4'-hydroxytamoxifen, but not ICI 164,384, induced reporter gene activity in cells transien tly transfected with pF1. E-2 induced reporter gene activity in MDA-MB -231 breast cancer cells transiently cotransfected with pF1 and wild-t ype ER or variant ER in which the DNA-binding domain was deleted (HE11 ); plasmids expressing N-terminal or C-terminal domains of the ER cont aining activator function-1 or -2, respectively, were inactive in thes e assays. In contrast, only wild-type ER mediated 4'-hydroxytamoxifen- induced activity. Induction of c-fos protooncogene expression by E-2 i n MCF-7 cells is dependent on the formation of a transcriptionally act ive ER/Sp1 complex that binds to a GC-rich enhancer element.